Phasing-in of vitrification into routine practice: why, how, and what.

Objective: To evaluate and compare the laboratory and clinical outcomes of vitrification with slow-freezing method for cryopreservation of embryos and blastocysts in an in-vitro fertilisation programme.

Design: Retrospective analysis of all the 104 cycles of frozen embryo and blastocyst replacements from 2003 to 2008 and all the 149 cycles with embryos or blastocysts for vitrification from 2006 to 2009.

Setting: Hospital-based Licensed Assisted Reproduction Treatment Centre in Hong Kong.

Participants: All participants having frozen embryos or blastocysts transfer from 2003 to 2008.

Interventions: Surplus embryos and blastocysts after fresh transfer were cryopreserved by vitrification method.

Main Outcome Measures: Cryosurvival rate after freeze-thawing of early cleavage embryos and blastocysts by the two cryopreservation methods of slow-freezing and vitrification, and the pregnancy rate, implantation rate, delivery rate and live-birth rate achieved.

Results: Cryosurvival rates of both vitrified blastocysts (79%) and early cleavage-stage embryos (88%) were significantly higher, as compared with the slow-freezing groups (57% and 72% respectively, both P<0.05). Pregnancy rates, delivery rates, and implantation rates were all significantly higher with vitrification regardless of the embryo types. Both implantation and live-birth rates were higher (31%, odds ratio=14 and 27%, odds ratio=11, respectively) per vitrified blastocyst transferred as compared with slow-freezing (both 3%).

Conclusion: Vitrification improved clinical outcomes of both frozen embryos and especially blastocyst transfers. It conferred upon both blastocysts and embryos better developmental potential after the vitrify-thaw procedure.